The present invention relates to underwater pelletizers, and more particularly to an improved extrusion die and die assembly for use in an underwater pelletizer.
Pelletizers have been known and used for a number of years. They are used to process rubber compounds, and molten thermoplastics and other polymers into pellets, which may in turn be used in other processes to manufacture various plastic materials.
In an underwater pelletizer, molten plastic is typically extruded through orifices in a die, forming continuous strands which are cut by knives at the die""s cutting face. The cutting takes place underwater in a water chamber or housing. Plastic pellets are formed which cool and harden in the water contained in the water chamber. Typically, the water in the water chamber is much cooler than the molten polymer, allowing very quick cooling of the polymer and quick solidification of the pellets.
Typically, an underwater pelletizer is constructed so that a constant stream of water passes over the die""s cutting face, conveying the hardened pellets along to equipment which separates out the pellets from the water and dries them.
It is very important that the pellets formed by such a pelletizing process be uniform in size and shape, and that they be made to a specific geometry. There are, however, a number of difficulties which may be encountered in trying to form uniform pellets.
To form uniform pellets of a specific geometry, the orifices in the extrusion die must remain free and clear of solidified polymer material. If partially plugged, an orifice will allow only a small or irregular strand of polymer through it; this leads to small, irregularly-shaped pellets. If an orifice is completely clogged, of course, no polymer may pass through it, and no pellets are formed, decreasing production of the pelletizer.
The rate of solidification of a polymer exiting an extrusion die in an underwater pelletizer depends upon the nature of the polymer and also on the temperature of the water in the water chamber and the speed of its flow. For any particular polymer, of course, the rate of solidification will depend mostly upon the water temperature. For some polymers, the water temperature must remain fairly cool to cause the polymer to solidify quickly. However, the die plate must not be allowed to become so cooled by the passing water that the polymer solidifies before it has exited the extrusion orifices.
In respect of other polymers, a very hot water temperature is required to keep the polymer from solidifying before exiting the extrusion orifices. Of course, high water temperatures can be hazardous if the water chamber bursts or otherwise leaks.
Rather than attempting to controlling solidification of polymer material merely by adjusting water temperatures, the die plates in underwater pelletizers are typically heated. Past efforts at heating pelletizer dies have focussed upon heating of the die plate by steam or hot oil, as shown in U.S. Pat. No. 4,123,207, which issued in 1978. Such steam and oil-heated die plates are costly, complicated, and require the addition to the pelletizer of a separate complicated system for heating the steam or oil. They are also hazardous.
Other die plates aimed at solving the problem of orifice clogging have employed complicated xe2x80x9csealed heat transfer tubesxe2x80x9d, as shown in U.S. Pat. No. 5,629,028. These tubes allegedly transfer heat from the molten polymer to the die. They are problematic since they are difficult to adjust for different polymers, which, as noted above, have different properties.
Still other prior art die plates are heated by inserting electric heating rods radially into chambers formed around their circumference, as shown in U.S. Pat. Nos. 4,621,996 and 5,403,176. At least one of the drawbacks of these systems is that the heat is concentrated at a limited number of points on one side of the die hole pattern, and not around the entirety of each of the orifices. To heat the portion of the orifice furthest away from the heating rod enough to prevent clogging, one might increase the heat in the heating rods, but this can cause polymer degradation nearest the heating rod.
There remains, accordingly, a need for an improved extrusion die having a heating system which provides better heating to the extrusion orifices, and to the entirety of each of them, without being of such complicated construction nor as dangerous as currently-employed systems.
The present invention is an extrusion die, and die assembly, for use in an underwater pelletizer. More particularly, the die preferably comprises a die plate having an upstream extruder face and a downstream cutting face, an inner annular groove formed in one face of the die plate, an outer annular groove formed in one face of the die plate, a plurality of spaced-apart extrusion orifices extending through the die plate from the extruder face to the cutting face, the orifices arranged in a pattern occupying a space between the inner and outer annular grooves and a coiled heating element contained within each of the inner and outer annular grooves, the coiled heating elements having leads extending outwardly from the die plate for connection to an electrical source.
In one embodiment of the invention, the pattern of extrusion orifices is a circular pattern, and the inner and outer annular grooves and the circular pattern of extrusion orifices are all concentric. In another embodiment, all are coaxial with the central axis of the die plate.
A thermally-conductive paste may be contained within each of the inner and outer annular grooves, occupying the volume not occupied by the heating coils, to assist heat transfer. Also assisting heating of the die are a plurality of electrical heating rods occupying chambers arranged radially in the die plate, the chambers extending inwardly from the circumference of the die plate.
The die assembly of the present invention comprises the die described above and also a die cone attached to the central portion of the extruder face of the die plate, the die cone covering and enclosing the inner annular groove but not the extrusion orifices. Also, an adaptor for attaching the die plate to the extruder may be attached to the die plate, the adaptor when bolted to the die plate covering the outer annular groove but not the extrusion orifices. In another embodiment, the die assembly can further comprise a circular flange attached to the cutting face of the die plate, the flange allowing said die plate to be attached to said water chamber, a circular cap attached to the cutting face of said die plate, and thermally-insulative gaskets fitted between the die plate and both the flange and cap.
In yet another embodiment of the invention, a die plate is provided having a plurality of coiled heaters.